When lifting loads of a substantial size it is a requirement that the orientation of the load is controlled. Examples of such loads may be wind turbine blades, towers, prefabricated concrete elements for buildings, etc.
Due to the inertia of the load the load may maintain its position in space even when the bearing wire is rotated, i.e. if the crane boom is rotated. Therefore an uncontrolled load may potentially impact the crane boom or impact personnel or structures close by. It is therefore a requirement that the load is controlled. For example by attaching a tagline to the load for applying a torque to the load about the bearing wire. The torque may be may be clockwise or counter clockwise.
Due to the size of the loads it is unsafe for personnel to handle the taglines and to apply the torque to the load about the bearing wire.
From WO 2008/061797 a method and a lifting device for mounting a wind turbine blade to a wind turbine hub are known. The blade is suspended in a substantially horizontal orientation from a bearing wire provided by a crane having a crane boom. The bearing wire is attached to the lifting device, which is attached to the blade. The blade is lifted from a position on the ground and to a position, where the blade may be mounted to the hub. In order to avoid that the blade rotates about the bearing wire the blade is connected to control wires, which run via pulleys on the crane boom to a winch arrangement at the bottom of the crane boom. The system applies one winch for controlling the orientation of the load and another manually operated winch for rotating the load.
Although the system of WO 2008/061797 has been successfully implemented there are some disadvantages of the system. A major disadvantage is that the crane needs to be modified. This normally requires approving from the crane manufacturer in order to guarantee the required functioning of the crane. The cranes are very expensive to operate and are only used for a short duration. A purpose modified crane would have much idle time, alternatively the pulleys, the winch arrangement and the control wires would have to be removed from the system of WO 2008/061797 between lifting operations at one site to free the crane for other uses.